The present invention relates to a fail-safe thermostat. More particularly, the present invention relates to an automotive thermostatic fluid control valve adapted for use in an engine coolant system and configured to have a fail-safe configuration for permanently opening a radiator conduit upon failure of the thermostat to reduce the likelihood of overheating the engine.
It is well known to provide a thermostatic fluid control valve or thermostat positioned in the coolant system of an internal combustion engine to control the volume of fluid flow through the heat exchanger or radiator as fluid circulates in the coolant system of the engine. Therefore, the thermostat maintains the coolant fluid circulating through the coolant system of the engine at a desired temperature. Conventional thermostats include a stationary valve member which is secured within a conduit of the cooling system. The stationary valve member has an annular valve seat forming a fluid port or opening. The thermostat also includes a movable valve member having a closure portion for engaging the valve seat to close the opening. A thermally responsive actuator is provided for moving the movable valve member relative to the stationary valve member in response to changes in temperature of the coolant fluid within the cooling system. As the temperature rises above a predetermined level, the actuator moves the movable valve member away from the valve seat to permit the coolant fluid to circulate through the radiator to cool the coolant fluid. Therefore, the thermostat maintains the coolant temperature substantially constant during operation of the engine.
It has long been recognized in the automotive industry that a significant cause of engine failure is overheating. One cause of engine overheating is the failure of the thermostat to open to permit coolant fluid to flow to the radiator. This failure of the thermostat to open is most often caused by the loss of the expansive material in the power element or thermally responsive actuator, which can occur for any number of reasons. When the actuator fails, the movable valve member is not moved away from the valve seat when the engine coolant temperature rises above the predetermined temperature. It is understood, however, that the thermostat may fail for other reasons. When the thermostat does not actuate, the pressure of the coolant fluid and the force of the thermostat operating spring cause the valve to remain in a closed position. A typical thermostat therefore fails in a closed valve or engine overheat mode.
One object of the present invention is to provide a fail-safe thermostatic control valve that will permit coolant flow to the radiator upon failure of the actuator under engine overheat conditions.
Another object of the present invention is to provide a fail-safe thermostat which is capable of withstanding an overheat incident which is not caused by failure of the thermally responsive actuator and remain fully functional.
Yet another object of the present invention is to provide a fail-safe thermostat that will remain in the open valve position, if there is a failure of the thermally responsive actuator, to provide an indication that replacement of the thermostat is necessary.
Fail-safe thermostats are generally known. See, for example, Kitchens U.S. Pat. No. 4,883,225. The Kitchens '225 patent discloses a disk valve secured to a cylindrical body by a fusible alloy. When the normal operating temperature range of the engine is substantially exceeded, the fusible alloy melts, thereby releasing the main disk valve from the cylindrical body. The main disk valve is then forced upwardly by a compression spring to permit full coolant flow through the radiator passageway. A problem associated with using a fusible alloy such as the fusible alloy disclosed in the Kitchens '225 patent is that the fusible alloy must resolidify somewhere inside the coolant system of the engine when the temperature drops below the melting point of the fusible alloy. In addition, it is possible for the fusible alloy to melt even though the thermally responsive actuator and the rest of the thermostat are functioning properly due to any of a number of other causes of overheat.
Another example of the use of a fusible material to provide a fail-safe oil flow control apparatus is illustrated in Duprez U.S. Pat. No. 4,537,346 owned by the assignee of the present invention. The Duprez '346 patent directs the flow of oil through the heat exchanger upon melting of the fusible material.
Conventional fusible alloy fail-safe thermostats must have an alloy melting temperature set above the normal operating range of the engine. Therefore, the engine temperature must exceed the normal operating temperature and enter the overheat range before the fusible alloy will melt. This overheat may cause damage to the engine. Another disadvantage of fusible alloy fail-safe thermostats is that the thermostat will fail as a result of any engine overheat condition, regardless of the cause (low coolant, etc.). Advantageously, the fail-safe thermostat of the present invention provides a two step operation which permits the fail-safe temperature to be set within the normal operating range. Two conditions must be met in order for the thermostat of the present invention to fail. First, the temperature must exceed the preset fail-safe temperature, and second, the thermally responsive actuator of the thermostat must be inoperative. Therefore, as long as the actuator is functioning properly, the fail-safe thermostat of the present invention will not fail. This feature prevents failure of the thermostat due to an overheat condition which is not caused by failure of the thermally responsive actuator.
According to one aspect of the present invention, a fail-safe thermostat apparatus is provided for controlling fluid flow through an engine cooling system in response to changes of an ambient temperature surrounding the apparatus. The apparatus includes a stationary valve member configured to be positioned between conduit components of the engine cooling system. The stationary valve member is formed to include a flange ring and a valve seat configured to define an opening into a conduit leading to the radiator. The apparatus also includes a movable valve member for engaging the valve seat to block fluid flow into the conduit through the opening, an operating spring for biasing the movable valve member to a closed position against the valve seat of the stationary valve member, and a thermally responsive actuator coupled to the stationary valve member and to the movable valve member to move the movable valve member to an open position away from the valve seat when the ambient temperature exceeds a first predetermined temperature to permit fluid flow into the conduit. The apparatus further includes a bimetallic frame member having at least two fingers configured to engage the flange ring or other means of attachment of the stationary valve member to couple the frame member to the stationary valve member. The frame member also has a base configured to compress and hold the operating spring in engagement with the movable valve member. The fingers of the frame member are configured to move radially inwardly and disengage from the flange ring of the stationary valve member when the ambient temperature exceeds a second predetermined temperature greater than the first predetermined temperature. The second predetermined temperature is preferably set at about a maximum operating temperature of the engine. The apparatus still further includes a fail-safe spring located on an opposite side of the movable valve member from the operating spring. The operating spring applies a larger biasing force to the movable valve member than the fail-safe spring so that the movable valve member is normally closed. The fail-safe spring biases the movable valve member away from the valve seat to move the movable valve member to the open position upon disengagement of the frame member from the stationary valve member. The movable valve member is configured to block radially inward movement of the fingers of the frame member when the movable valve member is in the open position, thereby preventing disengagement of the frame member from the stationary valve member of a functional thermostat.
In the illustrated embodiment, the thermally responsive actuator includes a movable stem. The stationary valve member includes a flange trapped between the conduit components of the engine cooling system and a bridge for coupling the stem of the thermally responsive actuator to the stationary valve member. The fail-safe spring is located between the thermally responsive actuator and the bridge for forcing the movable valve member to the open position upon disengagement of the frame member from the stationary valve member to permit fluid to flow through said opening.
Also in the illustrated embodiment, the bimetallic frame member includes a first metal material having a first coefficient of thermal expansion located radially outwardly from a second metal material having a second coefficient of thermal expansion. The first coefficient of thermal expansion is greater than the second coefficient of thermal expansion so that the at least two fingers of the frame member move radially inwardly to disengage the fingers of the frame member from the flange ring when the ambient temperature exceeds the second predetermined temperature.
In another illustrated embodiment, the movable valve is coupled to a first end of the operating spring and the bimetallic frame member is coupled to a second end of the operating spring to prevent loose parts from entering the cooling system of the engine upon disengagement of the frame member from the stationary valve member. A bypass valve may be coupled to the thermally responsive actuator for closing a bypass conduit of the cooling system in response to movement of the thermally responsive actuator.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.